Progesterone plays a central coordinate role in regulation of reproductive events associated with establishment and maintenance of pregnancy including ovulation, uterine and mammary gland development and tumorigenesis and neurobehavioral expression associated with sexual responsiveness. Although most of the downstream molecular and cellular mechanisms by which progesterone exerts these effects are unclear, they are mediated by interaction with intracellular proteins termed progesterone receptors (PR) whose function is to act as ligand activated transcription factors to regulate the expression of specific programs of reproductive target genes. Two PR proteins, termed A and B, that arise from a single gene have been identified in most mammalian species and null mutation of both proteins leads to pleiotropic reproductive abnormalities. Substantial evidence has accumulated in recent years to demonstrate that the A and B proteins display different tissue specific transcription regulatory properties in response to progestin agonists, antagonists and activation of cellular phosphorylation pathways when tested in tissue culture cells. These observations lead us to hypothesise that the physiological responses to progesterone are mediated by the combinatorial actions of two functionally distinct proteins, each of which acts in a differential tissue specific manner to influence the expression of a specific repertoire of target genes. The hypothesis predicts that alterations in the cellular composition of PRs may have significant impact on overall reproductive tissue responsiveness to progesterone and its antagonists including tumorigenic responses of the uterus and mammary gland to PR. To test these predictions we have introduced subtle mutations into the mouse PR gene to selectively ablate expression of the PR A or B proteins and we have generated two novel mutant mouse lines that express only the PRA or PRB proteins from the PR gene. During the second phase of the project, we will use these mutant mouse lines to examine the selective contribution of the PR A and B proteins to PR dependent reproductive function in four areas; 1) ovarian follicular development and fuction; 2) the differentiative and antiestrogenic activities of PR in the uterus; 3) the proliferative and differentiative functions of PR in the mammary gland; and 4) the ligand dependent and independent mechanisms of activation of sexual behaviors mediated by PR in the ventromedial hypothalamus.